The invention relates to the art of cermet preparation and especially to the preparation of unidirectionally solidified metal-metal oxide composites by internal zone growth.
Directionally solidified metal-metal oxide composites have been prepared in the prior art by inductive heating of pressed metal-metal oxide powder compacts by internal zone melting such as is described in commonly assigned U.S. Pat. No. 3,796,673 issued to G. W. Clark et al. Mar. 12, 1974, for "Method of Producing Multicomponent Metal-Metal Oxide Single Crystals," which is incorporated herein by reference. According to the internal zone growth technique, a compact, typically cylindrical, is prepared by pressing a mixture of metal and metal oxide powders in a conventional manner. The compact is sintered in some cases to increase the density. The compact is placed in an induction furnace within a radiofrequency induction coil oriented with the cylindrical compact coaxial with the coil. The compact typically had to be heated to above about 1500.degree. C. to increase the electrical conductivity sufficiently to permit coupling of the magnetic and electric fields to the compact, thereby providing RF induction heating. Normally, the induction heating is performed in an induction furnace containing susceptors which radiantly preheat the compact to the required temperatures. After the compact has been preheated sufficiently, the susceptors are withdrawn and the sample is heated directly by the RF induction coil. The induction heating process is capable of melting a zone within the compact while maintaining a solid "skin" of the compact to contain the melt. The induction coil current is normally increased until a molten zone forms within the compact. The formation of the molten zone can be detected by monitoring the surface temperature of the compact and the RF generator grid and plate currents.
The cylindrical sample is normally oriented vertically and, after the molten zone is established, the sample is raised or lowered relative to the localized radiofrequency field. As the sample is moved, solid changes into liquid ahead of the molten zone and liquid solidifies behind the molten zone. The solidified zone is typically single crystals of oxide containing discrete metal rods. The amount of metal relative to the oxide in the solidified composite is a function of the metal and oxide compositions of the compact and the temperature of the molten zone. If the molten zone is heated no higher than the eutectic temperature, the metal and oxide in the composite will be in the eutectic proportions. After the compact has been passed through the induction heating coils, it is cooled and sectioned to provide single oxide crystals having extended metal rods. Such directionally solidified cermet composites may be useful as MHD electrode materials, gas turbine components, electron emitters, high temperature valve seats, and hard tool and grinding materials.
During the formation of the molten zone within the compact, the power level of the induction coil must be controlled to prevent catastrophic melting of the external solid surface. The thickness of the skin is also a function of the frequency and the thermal conductivity, melting temperature, magnetic permeability, size and electrical conductivity of the compact. If the frequency is too high, melting occurs on the surface rather than in the interior. If the frequency is too low, a heating instability can result in melting of the surface and escape of the melt. Control of the molten zone is further complicated in metal-metal oxide mixtures by the fact that the composition (and hence the thermal and electrical conductivity) of the molten zone varies due to unavoidable heating fluctuations. Some cermet composites, for example those employing Cr.sub.2 O.sub.3 or (Cr,Al).sub.2 O.sub.3, have proven to be practically impossible to prepare by conventional internal zone growth directional solidification techniques because of the extreme dependence of electrical conductivity on the temperature of the oxide.